During the 2007 fall ragweed allergy season, participants will be observed and be asked to keep a diary of their asthma symptoms and medication use. In the spring of 2008, participants will be randomly assigned to receive six weekly doses of either the experimental medicine or placebo. Participants will be observed again for the 2008 fall ragweed season. Another series of three weekly doses of the assigned study treatment will be given in spring 2009, with observation through the 2009 fall ragweed allergy season and possibly the 2010 season. During the observation periods, participants will be asked to track their asthma symptoms and medication use and report this information to study staff. Study visits will be weekly during the spring and biweekly during the ragweed season, with a maximum of 16 visits per year. Allergy and lung tests as well as blood and urine collection will occur at selected study visits; these tests are designed to measure the participants' immune response.